Our objective is to investigate the effect of processes in the human accommodation system on several different aspects of monocular space perception in visually-normal individuals. Primary emphasis will be on defining the relationship between the steady-state accommodative level and the monocular perception of distance with tonic accommodation (the assumed perceptual reference point for the system) either at its habitual level or biased to determine the effect of accommodative hysteresis on both apparent distance and other distance-related percepts. Four experiments will be conducted: (1) the relationship between the accommodative response and perceived distance will be determined at optical distances from 2 to 6 diopters for each of three targets (contourless surface, cross with blank central field, reduced Snellen chart) which differ in their accommodative stimulus effectiveness (poor to excellent); (2) experiment 1 will be repeated at one accommodative stimulus level (3D) before and after biasing tonic accommodation by sustained focus at either the nearpoint or farpoint of accommodation to determine the effect of accommodative hysteresis on the blur-driven accommodation/perceived distance relationship; (3) the influence of accommodative hysteresis-induced changes in distance registration on perceived target velocity (2.0, 5.0 deg/sec) will be assessed when viewing a midline target at near (33 cm); and, (4) the influence of accommodative hysteresis-induced changes in distance registration on position constancy will be determined when viewing a midline target at near (33 cm). In each experiment, a Hartinger optometer will be used for measurement of tonic and/or blur-driven accommodation. Apparent distance and velocity will be measured by having the subject adjust a thimble-like slide with the unseen hand to match the spatial location and rate of translation of the visual target, respectively. Position constancy will be measured in terms of the physical motion of a target concomitant with lateral displacement of the head required for apparent stationarity.